1. Field of the Invention
The present invention relates to a liquid crystal display (LCD). Particularly, the present invention relates to a liquid crystal display in which individual pixel electrodes are provided for each of the pixels.
2. Related Background Art
A liquid crystal display of a matrix type used in an image information processing apparatus such as a video camera recorder or a television is driven by a drive system which is either a simple matrix drive system or an active matrix (AM) drive system. In an active matrix drive system, a switching active element is provided for each pixel so as to prevent a signal from being input to a pixel electrode during a non-select period thereby preventing a cross-talk phenomenon which might occur between scanning lines with a simple matrix drive system.
FIG. 1 shows a circuit equivalent to a circuit corresponding to one pixel of a liquid crystal display using an active matrix drive system. The circuit includes a liquid crystal layer 2 having a capacitance C.sub.LC, a storage capacitance element 4 provided in parallel with the capacitance C.sub.LC and having a capacitance C.sub.ADD, a metal-oxide semiconductor (MOS) transistor 6 serving as a switching active element, a signal line 8 for supplying driving voltage V.sub.S to the liquid crystal layer 2, and a gate line 10 for controlling a gate voltage V.sub.G of the switching transistor 6.
In such an active matrix liquid crystal display (AM-LCD), a parasitic capacitance C.sub.GD exists between the gate and the drain of the switching transistor 6. When the gate voltage V.sub.G shifts from a high level to a low level, the parasitic capacitance C.sub.GD causes a reduction in a voltage V.sub.LC applied to the liquid crystal, as shown in FIG. 2. That is, a voltage swing occurs. Such a reduction .DELTA.V.sub.LC is expressed by the following formula: EQU .DELTA.V.sub.LC =[C.sub.GD /(C.sub.GD +C.sub.LC +C.sub.ADD)].multidot..DELTA.V.sub.G
where .DELTA.V.sub.G represents a change in the gate voltage V.sub.G
Image quality decreases if the swing .DELTA.V.sub.LC is great. Therefore, in order to improve image quality, it is preferable that the storage capacitance C.sub.ADD be as great as possible so that the swing .DELTA.V.sub.LC can be minimized. However, if the storage capacitance element 4 is formed by using an opaque material, such as that used in a gate line or an active layer of a switching transistor, so as to achieve a large storage capacitance C.sub.ADD, the proportion of an opaque portion relatively increases, causing a corresponding reduction in the open area ratio. The use of a large storage capacitance C.sub.ADD is also disadvantageous in that it is necessary to increase the charging and discharging ability of the switching transistor 6, thereby requiring the use of a large-size transistor. A reduction in the open area ratio may be prevented by using a capacitance element made of an indium-tin oxide (ITO-ITO) material. However, since ITO can cause a relatively high resistance, it is necessary to reduce resistance by using an opaque metal wire such as an Al wire. Thus, the above method cannot be a fundamental solution to the problem of open-area ratio reduction.
There are capacitances other than the above C.sub.GD that can add to the parasitic capacitance; these include a capacitance C.sub.GP between the gate line 10 and the pixel electrode 16, and a capacitance C.sub.SP between the signal line 8 and the pixel electrode 16. These capacitances C.sub.GP and C.sub.SP can cause reductions in the voltage V.sub.LC applied to the liquid crystal as does the capacitance C.sub.GD. The capacitance C.sub.GP causes a change corresponding to a change in the potential of the gate line 10, and the capacitance C.sub.SP causes a change corresponding to a change in the potential of the signal line 8.
The deterioration of display performance caused by these capacitances C.sub.GP and C.sub.SP becomes more and more conspicuous as displays are required to have higher and higher definition and gradation levels and pixels become more and more minute in order to realize televisions or the like adapted for a hi-definition system.
However, if this problem is to be overcome by increasing the space between the gate line and the pixel electrode and the space between the signal line and the pixel electrode, this necessitates a substantial increase in cell area. Since such increase contradicts the demand for higher definition, the above method is not suitable for the intended purpose.